Rendering aluminum and aluminum alloy surfaces corrosion resistant



Patented Feb. 8, 1949 RENDERING AND AL ALULIINUM UMINUM ALLOY SURFACES.CORROSION RESIST- ANT Walter R. Meyer, Hamden, Conn., assignor. bymesne assignments, to Enthone, Inc., New Haven, Conn, a corporation ofConnecticut No Drawing. Application September 14, 1944, Serial No.554,147

Claims. (01. 148-620) This invention relates to a method for produc-'ing corrosion-resistant films on aluminum and aluminum alloy surfacesand encompasses compositions employed in the practice of the method. Theinvention is particularly valuable in that it eliminates the necessityfor anodizing metal surfaces comprising aluminum.

It is known to coat aluminum or aluminumcontaining alloys by means of anaqueous solution of an alkali metal chromate and an alkali metalcarbonate. The coatings provided by such solutions give only a fairmeasure of protection to surfaces of pure aluminum or non-cuprous alloysof aluminum while the protection afforded surfaces of copper-aluminumalloy is very poor.

I have discovered that the inclusion of an alkali metal chlorite in thesolution markedly improves the quality of the protective coating as tosurfaces of pure aluminum or non-cuprous alloys of aluminum; also, thatthe solution comprising an alkali metal chlorite will give asatisfactory coating on a copper-aluminum alloy surface.

In the preparation of a coating solution according to my invention, Imay supply part or all of the chromate radical in the solution throughthe addition of an alkali metal dichromate since the alkali metaldichromate will immediately react with the alkali metal carbonate toform alkali metal chromate. I have found that the chromate concentrationin the solution, calculated as mono-chromate, is most suitably keptbetween 1 and 20 g./l. I have also found it advantageous v to maintainthe concentration of alkali metal chlorite within that same range. Themost advantageous concentration of alkali metal carbonate appears to beinterrelated with the concentrations of the other components. Usuallythe carbonate concentration is best maintained,

between 8 and 50 gf/l. It is my preferred practice to use sodium saltsin the preparation of the coating solution, but the corresponding saltsof other alkali metals, for example potassium salts, may be used.

The coating operation is most suitably carried out by immersing the workpiece in the solution. Any operating temperature promoting the formationof a corrosion resistant film on the aluminum or aluminum alloy surfacemay be employed, but I prefer to operate the solution or bath at atemdesired film is a function of the concentration of '2 peraturebetween 200 F and its boili I ng oin The color of the coatings producedmay var from a purplish iridescent color to a dark greenish gray colordepending upon the com the surface contacted with the solutio ffi of Incoating surfaces of substantially pure aluminum or of alloys of aluminumwith manganese magnesium, or chromium, I preferably employ a bath ofpproximatel th 1 trations: y e onowmg en- Sodium chlorite gJ15. Sodiumchromate 5-6 Sodium carbonate 20 In treating copper-aluminum alloysurfac I have found that a bath containing from 4 t 16 g./l. of sodiumchlorite, 4 to 5 g./l. of sodium chromate, and from 12 to 16 g./l.v ofsodium carbonate gives particularly good results. Such baths may, withadvantage, be prepared by dissolving in water a solid composition individed form consisting essentially of the three salts in the relativeproportions desired in the bath.

My investigations indicate that whenever the chromate concentration isincreased, it is advisable to increase the carbonate concentration.

It is preferable in all cases that the aluminum or aluminum alloysurface before treatment according to my invention be cleaned with analkali that etches the surface to provide clean active metal forcoating. An aqueous solution of trisodium phosphate or caustic sodaheated to about 180 to 212 F. has been found suitable for the cleaningoperation. In preparing the tri-sodium phosphate solution, I usuallyemploy about 6 oz. of the tri-sodium phosphate for each gallon of water.In preparing the caustic solution, I customarily use about lb. ofcaustic soda for each gallon of water. Following the treatment with thewarm alkaline solution, the surface should be dipped in strong nitricacid (50 or stronger) to remove any copper or iron smut. The work isthen ready for immersion in the oxidizing solution containing achlorite, chromate, and carbonate of an alkali metal.

In general, the time required to produce the the bath and of theoperating temperaure.

of solutions of low concentration than in the case or solutions ofrelatively higher concentration to produce a coating in the same lengthof time. Certain aluminum alloy surfaces may require slightlyhighertemperatures and concentrations than surfaces of pure aluminum.

The invention is further illustrated by the following examples which arenot to be considered as in any way limiting the scope thereof.

Example I A composition containing 1 part by weight of sodium chlorite,1.1 parts by weight of sodium chromate (about 1.013 parts by weight ofsodium dichromate, if used) and 4 parts by weight of sodium carbonatewas dissolved in sufficient hot water (200 F.) to give an aqueoussolution in which the concentrations ofthe salts were approximately asfollows:

Grams per liter Sodium chlorite 5 Sodium chromate 5.5

Sodium carbonate 20 A strip of commercially pure aluminum was immersedin the solution and allowed to remain therein for a period of aboutminutes. This resulted in the production of a film on the aluminum stripwhich withstood 263 hours of a salt spray at 95 F. without a sign ofcorrosion.

Example II The coating solution was prepared as in Example I. A strip of38 aluminum was immersed in the solution for 15 minutes. The coatingproduced on the strip did not break down when subjected for 263 hours toa 20% salt spray at 3S aluminum is an alloy of aluminum with manganese,containing 1.25% of the latter metal.

Example III The coating solution was prepared as in Example I. A stripof 52S aluminum, an alloy of aluminum with magnesium and chromiumcontaining 2.5% and 0.25% respectively of these metals, was immersed inthe solution for 15minutes. The resulting film also withstood 263 hoursof a 20% salt spray at 95 F. without a sign of corrosion. v

Example IV A strip of 24S aluminum was immersed in the solution andallowed to remain therein for a period of about 15 minutes. The filmproduced on the strip did not break down after 40 hours of a 20% saltspray at 70 F.

245 aluminum contains 4.2% copper, 0.5% manganese and 1.5% magnesium,the balance being aluminum and impurities.

Example V One part by weight of a composition containing 1 part byweight of sodium dichromate, 4 parts 4 by weight of sodium chlorite, and3 parts by weight of sodium carbonate was dissolved in about 30 parts byweight of hot water (200 F.) to give a solution of the followingconcentrations:

Grams per liter Sodium chlorite 16 Sodium chromate 4.34 Sodium carbonate12 A strip of 24S aluminum was immersed in the solution and allowed toremain therein for a period of about 15 minutes. 48 hours of a 20% saltspray at 70 F. was required to break down the film produced on thestrip.

- Example VI One part by weight of a composition contain ing 1 part byweight of sodium dichromate, 2 parts by weight of sodium chlorite, and 4parts by weight of sodium carbonate was dissolved in about 36 parts byweight of hot water (200 F.) to give a solution of the followingconcentrations:

Grams per iiter Sodium chlorite 8 Sodium chromate 4.34 Sodium carbonate16 A strip of 24S aluminum was immersed in the so lution and allowed toremain therein for a period of about 15 minutes. 48 hours of a 20% saltspray at 70 F. was required to break down the film produced on thestrip.

It is to be understood that my invention is not restricted to the use ofsodium compounds but may be effectively practiced using other alkalimetal salts, for example potassium chlorite, potassium chromate and/ordichromate, and potassium carbonate.

A composition according to my invention contains from 1-20 parts of analkali metal chlorite, 1-20 parts of an alkali metal chromate ordichromate, and 8-50 parts of an alkali metal carbonate. Anhydrousmaterials in a state of fine subdivision are preferred for use in thepreparation of the composition. The materials may be mixed using anysuitable mixing device, for example an ordinary mixing drum of the typedesigned for the intimate admixing of fine powders.

My invention is not limited to coating articles of solid aluminum oraluminum alloys but may be used to coat surfaces of other materialswhich have been plated with aluminum or aluminum alloys when the coatingthickness is greater than 0.00005" and the aluminum content is greaterthan about 60%.

I claim:

1. The method of treating a surface composed of aluminum or an aluminumalloy having a content of aluminum greater than about 60%, the surfacehaving a thickness of at least 0.00005 inch when it represents acoating, which comprises contacting the surface with an aqueous solutionconsisting essentially of water and an alkali metal chlorite, an alkalimetal chromate and an alkali metal carbonate containing from 1 to 20grams per liter each of the chlorite and chromate and from 8 to 50 gramsper liter of the carbonate, the temperature and duration of thetreatment being such as to result in the formation of a corrosionresistant 'film on the surface.

2. The method of treating a surface composed of aluminum or an aluminumalloy having a content of aluminum greater than about 60%, the

surface having a thickness of at least 0.00005 inch when it represents acoating, which comprises contacting the surface with a hot aqueoussolution consisting essentially of water and sodium chlorite, sodiumchromate and sodium carbonate containing from 1 to 20 grams per litereach of the chlorite and chromate and from 8 to 50 grams per liter ofthe carbonate, the duration of the treatment bein such as to result inthe formation of a corrosion resistant film on the surface.

3. The method of treating a surface composed of aluminum or anon-cuprous alloy of alumlnum having an aluminum content greater thanabout 60%. the surface having a thickness of at least 0.00005 inch whenit represents a coating, which comprises contacting the surface with ahot aqueous solution consisting essentially of water and sodiumchlorite, sodium chromate and sodium carbonate containing about gramsper liter each of the chlorite and chromate and about 20 grams per literof the carbonate, the duration of the treatment being such as to resultin the formation of a corrosion resistant film on the surface.

4. The method of treating a surface composed of a copper-containingalloy of aluminum having a content of aluminum greater than about 60%,the surface having a thickness of at least 0.00005 inch when itrepresents a coating, which comprises contacting the surface with a hotI aqueous solution consisting essentially of water and sodium chlorite,sodium chromate and sodium carbonate containing from about 4 to about 16grams per liter of the chlorite, about 4 to 5 grams per liter of thechromate, and from about 12 to about 16. grams per liter of thecarbonate, the duration of the treatment being such as to result in theformation of a corrosion resistant film on the surface. Y

5. The method of claim 3 where the treating solution is maintained at atemperature between 200 F. and its boiling point.

6. The method of claim 4 where the treating solution is maintained at atemperature between 200 F. and its boiling point.

7. A solid composition in divided form adapted for use in aqueoussolution to produce a corrosion resistant film on a surface composed ofaluminum or a non-cuprous alloy of aluminum having a content of aluminumgreater than about 80%, consisting essentially of an intimate mixture ofsodium chlorite, sodium chromate and sodium carbonate in approximatelythe proportions given below:

8. A solid composition in divided form adapted for use in aqueoussolution to produce a corrosion l0 resistant film on a surface composedof a copper-aluminum alloy having a content of aluminum greater than60%, consisting essentially of an intimate admixture of sodium chlorite,so-

dium chromate and sodium carbonate in approx- 15 imately the proportionsgiven below:

Parts by weight Sodium chlorite 4to 16 Sodium chromate 4 to 5 Sodiumcarbonate 12 to 16 20 9. A solid composition in divided form consistingessentially of an intimate admixture of an alkali metal chlorite, analkali metal chromate an an alkali metal carbonate, the relative prointhe following indicated ranges:

Parts Alkali metal chlorite 1 to 20 Alkali metal chromate 1 to 20 Alkalimetal carbonate 8 to 50 Parts Sodium chlorite 1 to 20 Sodium chromate 1to 20 Sodium carbonate 8 to 50 WALTER R. MEYER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,838,633 Pacz Dec. 28, 19312,205,708 Zurbrugg June 25, 1940 2,364,993 Meyer Dec. 12, 1944 portionsof the three salts by weight being with-

